5.2 Answer the following questions:
(a) The earth’s magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?
(b) The earth’s core is known to contain iron. Yet geologists do not regard this as a source of the earth’s magnetism. Why?
(c) The charged currents in the outer conducting regions of the earth’s core are thought to be responsible for earth’s magnetism. What might be the ‘battery’ (i.e., the source of energy) to sustain these currents?
(d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth’s field in such a distant past?
(e) The earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km). What agencies may be responsible for this distortion?
(f) Interstellar space has an extremely weak magnetic field of the order of 10–12 T. Can such a weak field be of any significant consequence? Explain.
[Note: Exercise 5.2 is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]
(a) Earth's magnetic field changes with time. It takes a few hundred years to change by an appreciable amount. The variation in the earth's magnetic field with time cannot be neglected.
(b) Earth's core contains molten iron. This form is not ferromagnetic. Hence, this is not considered as a source of earth's magnetism.
(c) The radioactivity in the earth's interior is the source of energy that sustains the currents in the outer conducting regions of the earth's core. These charged currents are considered to be responsible for the earth's magnetism.
(d) Earth reversed the direction of its field several times during its history of 4 to 5 billion years. These magnetic fields got weakly recorded in rocks during their solidification. One can get clues about the geomagnetic history from the analysis of this rock magnetism.
(e) Earth's field departs from its dipole shape substantially at large distances (greater than about 30,000 km) because of the presence of the ionosphere. In this region, the earth's field gets modified because of the field of single ions. While in motion, these ions produce the magnetic field associated with them.
(f) An extremely weak magnetic field can bend charged particles moving in a circle. This may not affect the passage of charged particles.
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